Gelatin relief image

ABSTRACT

Photographic material comprising a support coated, in order, with a nonlight-sensitive gelatin layer partially cross-linked with disulfide linkages (e.g. thiolated gelatin) and an unhardened gelatin silver halide layer. After imagewise exposure, development of the silver halide layer and treatment with an alkaline mercaptan solution, the material is hot water-washed to form a relief image. Either a positive or a negative image may be obtained.

Unite States Patent Inventor John Peter Stonham lltord, Essex, England Appl. No. 832,020 Filed June 10, 1969 Patented Sept. 21, 1971 Assignee llford Limited llll'ord, Essex, England Priority June 12, 1968 Great Britain 27947/68 GELATIN RELIEF IMAGE 8 Claims, No Drawings [1.5. (31 96/36, 96/ 68 96/3 5 lint. 1C1 G031: 5/00 Field oli Search 96/35, 36, 3 6.3, 68

[56] References Cited UNITED STATES PATENTS 3,046,126 7/1962 Siis etal 96/363 3,145,104 8/1964 Oster etal. 96/363 Primary Examiner-Norman G. Torchin Assistant Examiner-John Winkelman Attorney-Cushman, Darby & Cushman GELATIN RELIEF IMAGE This invention relates to novel photographic material and to methods of processing such materials.

According to the first feature of the present invention, therefore, there is provided photographic material which comprises a support having coated on one side thereon, in order, a gelatin layer which is partially cross-linked by the presence therein of disulfide linkages and an unhardened gelatin silver halide emulsion layer.

It is preferred that there is present in the layer of partially crosslinked gelatin an inert pigment for example carbon black.

The utility of this photographic material lies in the fact that it has been discovered that when such material is treated with an aqueous solution of a mercaptan compound for a predeter mined period of time a selective decross-linking reaction of the gelatin takes place and the gelatin may easily be removed by washing. This discovery may be used to enable both negative and direct positive photographic records to be obtained using the photographic material of the present invention.

Thus according to a second feature of the present invention there is provided a method of producing an image of an original subject which comprises imagewise exposing the photographic material hereinbefore defined, developing the material in a nontanning silver halide developing solution and simultaneously or subsequently treating the material with an alkaline solution of a mercaptan for a predetermined period of time, and washing the material in water to remove the areas of decross-linked gelatin and the silver halide emulsion adherent thereto.

Either a positive or a negative image may be obtained by means of the method of the present invention. If the exposed material is kept in the presence of the mercaptan compound either during or after development for a relatively short period a negative image is obtained. If the exposed material is kept in the presence of a mercaptan compound, which is also a silver halide solvent, for a longer period a direct positive image is obtained. However if the material is kept in the presence of a mercaptan compound for too long a period no image at all is obtained as all the gelatin becomes decross-linked and removed during the washing step. The actual length of time during which the material should be kept in the presence of the mercaptan compound depends on a number of different factors, for example on the actual mercaptan used, on the concentration of the mercaptan in the aqueous solution, on the processing conditions such as temperature of solution used and on the thickness of both the silver halide and the gelatin layer. Thus as most mercaptan compounds are silver halide solvents it is usually possible by altering the period of time in which the exposed material is kept in the presence of a mercaptan compound either during or after development to obtain either a positive or a negative image. However it is necessary in all cases to carry out a prior test to determine the length of time required for the material to be in contact with the mercaptan compound.

In general it is preferred that the material after exposure is developed in a developing solution not producing a significant degree of tanning and subsequently is treated with an aqueous alkaline solution of a mercaptan compound.

For simplicity it is assumed in the following explanation of the process that the material has been exposed and developed in a nontanning silver halide developing solution prior to treatment with a mercaptan solution. It is thought that when the developed material is first treated with the aqueous alkaline mercaptan solution that the mercaptan penetrates more rapidly into those areas of the material where there is little or no developed silver. On reaching the underlying gelatin layer the mercaptan proceeds to decross-link those areas of the gelatin which underlie those areas in the emulsion layer which contain little or no developed silver. Hence if the material is removed from the mercaptan solution at this stage and washed a negative image of the original is obtained. However if the material remains in a mercaptan solution, which is a silver halide solvent, for a longer period of time the mercaptan begins to dissolve out the undeveloped silver halide in the emulsion layer. This causes the mercaptan in the corresponding underlying areas of the gelatin layer to tend to diffuse back into the emulsion layer and because the decross-linking reaction is reversible the reduction in the free mercaptan tends to cause the gelatin to become cross-linked once again. On the other hand the penetration of the mercaptan compound through the developed regions of the emulsions, although slower, suffers no such reversal and hence eventually these areas of the gelatin layer, which is preferably pigmented, become decrosslinked and may be removed by washing in water together with the overlying silver halide emulsion layer which contains the negative image in developed silver. This results in a positive image, the image being provided by the areas of pigmented gelatin layer remaining on the material. However if treatment with mercaptan solution is further prolonged then once all the silver halide has been dissolved by the mercaptan, the mercaptan can penetrate again to the underlying areas of the gelatin layer and decross-link all the gelatin layer. Thus on washing all the gelatin layer is removed together with all the overlying emulsion layer.

Another and much more complicated method of producing a negative image of an original subject comprises imagewise exposing the photographic material hereinbefore defined, developing the material in a nontanning silver halide develop ing solution, removing the undeveloped silver halide from the material by fixing it out, bleaching the developed silver to silver halide, treating the material with an aqueous alkaline solution of a mercaptan compound and washing the material in water thereby to remove the silver halide emulsion from the material and the gelatin from the unexposed areas of the material.

The final image areas in all cases are in relief compared to the nonimage areas and may be dyed, however as hereinbefore stated it is preferred that the photographic material of the present invention comprises, dispersed in the underlying gelatin layer, a colored pigment or a gelatin substantive dye. Preferably the color pigment is carbon black since by its use a dense black highcontrast image is obtained.

It is preferred that the photographic material of the present invention comprises thiolated gelatin which after coating on the material has been subjected to aerial oxidation. This oxidation promotes cross-linking of the adjacent thiol groups to form disulfide linkages, thus producing a layer of gelatin which has been partially cross-linked! by the presence of disulfide linkages therein. A particularly suitable thiolated gelatin for use in the present invention is Thiogel" which is manufactured by the Schwarz Bioresearch Inc.

Alternatively there can be used in the material of the present invention gelatin which has been partially cross-linked by the presence therein of disulfide linkages, this gelatin having been obtained by using the cross-linking agents described in our copending British Pat. application No. 27006/69 which was filed on 28th May, 1969.

In British Pat. application No. 27006/69 there are described gelatin cross-linking agents of the general formula:

R S -S -R where R and R are each selected from groups capable of reacting with gelatin so as to cross-link the gelatin.

A method of producing a layer of gelatin which is partially cross-linked by the presence therein of disulfide linkages is to treat an aqueous gelatin solution with a solution of a gelatin cross-linking agent of the general formula R -S -S -R' as hereinbefore defined and to coat the mixture on a support.

In Examples 7, 8 and 9, which follow, gelatin layers which have been partially cross-linked in this manner are used. The method for their preparation is set forth in the following Preparations.

PREPARATION 1 15g. thiomalic acid were dissolved in 200 ml. water, a trace of ferrous sulfate added and this solution cooled to 5 C. 7.75 ml. 22% hydrogen peroxide were then added slowly with vigorous stirring, the reaction mixture being cooled in ice. During the addition the temperature rose to 15 C.

After standing overnight the solution was acidified by the addition of ml. concentrated hydrochloric acid and then saturated with sodium chloride. 1t was then extracted four times with 50 ml. portions of ether. The ether extracts were dried over anhydrous sodium sulfate overnight and the ether distilled off under reduced pressure. The residue obtained (12 g.) was a white powder m.p. 161 C.

6 g. of this residue was then refluxed with 9 ml. acetyl chloride until all the solid had dissolved (about 2 hours) and the solution thoroughly cooled in ice. The crystalline product was filtered off, washed with ether and dried. Yield of dithiobis(succinic anhydride) 4.65 g. m.p. 108-1 10 C.

A 1% solution of the solid in acetone was prepared and added to 10% aqueous gelatin solution to give cross-linking agent/gelatin ratios of 0.5, l, 1.5 and 2.5%. The pH of the solutions were adjusted to 7 and they were diluted to give a gelatin concentration of 6%. The solutions were then coated on glass plates and after drying the plates were stored at 125 F. and 68% RH for three days to accelerate the cross-linking reaction. The dispersion temperatures of the gelatin coatings in water were then as follows:

k Addition m.p. (F)

PREPARATION 11 3.64 g. dithiodiglycollic acid and 1 1.12 g. p-nitrophenol were dissolved together in 15 ml. tetrahydrofuran. 8.24 g. dicyclohexyl carbodi-imide was dissolved separately in 10 ml. of the same solvent and the two solutions mixed at room temperature. After leaving to stand for 24 hours the dicyclohexylurea byproduct was filtered off, washed with tetrahydrofuran and the washings added to the filtrate. The latter was then evaporated to dryness under reduced pressure and the residue recrystallized from the minimum amount of boiling alcohol (about 100 ml.). After filtering hot the solution was cooled in ice and the product filtered off, washed with a little cold alcohol and dried. Yield of di-(p-nitrophenyl)dithiodiglycollate 4.4 g. of pale buffcrystals, m.p. 108-l 10 C.

The same test in gelatin coatings was carried out on this compound as described in Preparation 1, except that the crosslinking agent was dissolved in 2-ethoxyethanol and the coatings were stored at 125 F. and 68% for only one day. The dispersion temperatures of the gelatin in these coatings were as follows:

Table- Continued A piece of the coating containing 0.5% addition of crosslinking agent began to disperse in the 0.5% cysteine solution after 6 to 7 minutes at 120 F., whereas in 5% aqueous sodium carbonate the same coating required more than 30 minutes.

It is preferred that the aqueous alkaline solution of a mercaptan which is used to decross-link the partially cross-linked gelatin in the process of the present invention is an aqueous alkaline solution of cysteine because this compound is cheap, nontoxic and does not have a disagreeable odor.

By use of the photographic material and processes of the present invention and in particular of the photographic material when it comprises carbon black, a high-contrast image may readily be obtained. An advantage of these processes is that the photographic material need comprise only a relatively small amount of silver halide compared to the amount usually required to achieve an image of comparable density using silver alone as the image-providing material.

The processes of the present invention are equally suitable for the production of line and continuous tone images.

The following Examples will serve to illustrate the invention.

EXAMPLE 1 To ml. of 10% aqueous gelatin solution was added 100 m1. of a 15% aqueous dispersion of Sterling MTFF carbon black. (Sterling MTFF carbon black is a product of Cabot Carbon Limited). To this mixture was then added 250 ml. of a 4% solution of Thiogel. (Thiogel is a thiolated gelatin derivative manufactured by Schwarz Bioresearch Inc.). After diluting to 500 ml. the mixture was coated on a flexible support to give an optical density of about 4.5.

A coating was then applied of a slow chlorobromide emulsion to give a silver coating weight of 35 mgmldm The coating was stored for 3 days at 51 C. and 68% R.1-1. to accelerate the cross-linking reaction in the layer containing the Thiogel. At the end of this period the Thiogel had become sufficiently cross-linked to withstand immersion in hot water.

A piece of this coating was exposed to a line image and developed for 2 minutes in a 1-pheny1-3- pyrazolidone/hydroquinone based developer solution at 20 C. After 30 seconds in a stop bath and 1 minute wash it was soaked in a 0.5% solution of cysteine hydrochlorine in 5% aqueous sodium carbonate at 20 C. for 3 minutes. On washing in hot water to remove the emulsion layer and the softened regions of the pigment layer, a positive copy of the original was obtained.

A second piece of this coating was exposed and developed as before but after the stop bath the undeveloped silver halide was fixed out by soaking for 2 minutes in a 20% aqueous sodium thiosulfate. The coating was then washed for 2 minutes and the developed silver converted to silver bromide by soaking in a bleach bath containing 50 g. each of potassium bromide and potassium ferricyanide per litre. After a further 2 minute wash it was soaked for 3 minutes in the cysteine solution and washed in hot water as before, when a negative copy of the original was obtained.

EXAMPLE 2 The coating prepared as described in example 1 was ex posed as before and developed for 2 minutes in a l-phenyl-3- pyrazolidone hydroquinone developer at 20 C. After 30 seconds immersion in the stop bath and washing for 1 minute, it was transferred to a 0.5% solution of thiomalic acid (mercapto-succinic acid) in 5% aqueous sodium carbonate at 23 C. After 3 to 4 minutes in this solution the film was removed and washed in hot water to remove the emulsion layer and the softened regions of the pigment layer, giving a positive copy of the original.

EXAMPLE 3 The coating prepared as described in example 1 was exposed and developed as described in example 2. After immersion in the stop bath for 30 seconds and a 1 minute wash, it was soaked for 2 minutes in a 0.5% solution of thioglycollic acid in 5% aqueous sodium carbonate. It was then washed in hot water, and a positive copy of the original was obtained.

EXAMPLE 4 Clear acetate base was first coated with the following solutions to improve the adhesion of the pigment layer.

5 g. thiolated gelatin (Thiogel made by Schwarz Bioresearch Inc. and 1.25 g. salicylic acid were dispersed in a mixture of 70 ml. water and ml. methyl alcohol and a further 165 ml. methyl alcohol added with vigorous stirring. 325 ml. of acetone were then added also with vigorous stirring, and a further 25 ml. water to redissolve the small quantity of precipitated gelatin. The base was coated with this solution at 10 feet per minute at room temperature and dried.

The pigment layer coating solution was prepared as follows.

To 100 ml. 10% gelatin were added 100 ml. ofa 12% aqueous dispersion of carbon black (Sterling MTFF made by Cabot Carbon Ltd.) and 250 ml. 4% Thiogel solution. 3 ml. Teepol X solution were added as spreading agent and 10 ml. dimethyl sulfoxide as a mild oxidant to facilitate the crosslinking reaction (Teepol X is a mixture of secondary alkyl sulfates). The pH of the solution was adjusted to 6.7 and the volume diluted to 500 ml. Clear base previously treated as described above, was coated with this solution at feet per minute at 35 C. and dried to give a coating with an optical density of 3.2. The coating was then stored at 125 F. and 68% RH for 24 hours to facilitate the cross-linking reaction at the end of which time the coating was sufficiently hardened to withstand hot water (50 C.).

This coating was then coated with a slow chlorobromide emulsion to give a silver coating weight of 21 mgldm A strip of this coating was exposed to a step wedge and developed for 2 minutes in a Phenidone-hydroquinone developer at 68 F. (Phenidone is l-phenyl 3-pyrazolidone). After 1 minute immersion in a 2% acetic acid stop bath it was washed for 2 minutes and then immersed in a 0.5% solution of a cysteine hydrochloride in 5% aqueous sodium carbonate at 20 C. for 20 seconds, followed by washing in hot water to remove the emulsion layer and the solubilized regions of the pigment layer. A negative record of the original step wedge was obtained with a Dmax of 3.18 and a fog density of 0. l 6.

In comparison, a coating of the same emulsion alone on clear base with a silver coating weight of3l mgldm when exposed in the same manner and developed for 2 minutes in the same Phenidone-hydroquinone developer followed by con ventional fixation and washing, had a Dmax of 2.4 and a fog density of 0.05.

A second strip of the coating was exposed and developed as before and then immersed in a 5% aqueous sodium carbonate solution containing 3.14 g/litre cysteine hydrochloride at 20 C. for 2 minutes. On washing in hot water as before a positive record of the wedge was obtained with a Dmax of 3.2 and a fog density of 0.06.

EXAMPLE 5 A second pigment coating was prepared in exactly the same manner was described in example 4, except that the coating solution was applied at 30 C. On drying it had a density of 3.45.

After 24 hours storage at 125 F. and 68% RH a coating ofa slow chlorobromide emulsion was applied to give a silver coating weight of 22 mg/dm A piece of this material was exposed to a line image and developed for 2 minutes at 20 C. in a Phenidone-hydroquinone developer. After washing for 10 minutes, it was soaked for 1 minute in a 5% aqueous sodium carbonate solution contain ing 3.14 g./liter of cysteine hydrochloride. After washing in hot water as before a negative copy of the original was obtained.

EXAMPLE 6 A piece of the material used in example 5 was exposed to a step wedge and developed for 2 minutes as before. After 1 minute in a stop bath, it was washed for 5 minutes and then soaked for 40 seconds in a 5% sodium carbonate solution containing 0.6 g./liter N-acetylhomocysteine-thiolactone which has previously been converted to the free acid by dissolving in the theoretical amount of N.sodium hydroxide solution. After washing in hot water a negative record was obtained.

Similarly, a piece of the same material was exposed to a line image and after development and washing was soaked in a 5% sodium carbonate solution containing 1.5 g./liter N-acetylhomocystemethiolactone, converted to the free acid as before, for 2 minutes at 20 C. After washing in hot water a negative record was obtained.

EXAMPLE 7 To 200 ml. 10% gelatin solution were added, ml. of a 12% aqueous dispersion of Sterling .MTFF carbon black, 3 ml. Teepol X and 0.3 g. dithiobis (succinic anhydride) as prepared in Preparation I dissolved in 30 ml. acetone. The pH of the solution was adjusted to 7 and the volume adjusted to 500 ml.

This solution was coated on clear flexible base at 20 feet per minute and 35 C. When dry the coating has a density greater than 4, and it was then stored for three days at F. and 68% RH to accelerate the cross-linking reaction, after which it was sufficiently hard to withstand hot water.

It was then coated with a slow chlorobromide emulsion to give a silver coating weight of 24 gm ldm A piece of this material was exposed to a line image and developed as before. After 1 minute in a stop bath and 5 minutes washing, the material was soaked in a 5% sodium carbonate solution containing 3.14 g./liter cysteine hydrochloride for Zminutes at 20 C. After washing in hot water a positive record was obtained.

EXAMPLE 8 A piece of the material used in example 7 was exposed to a line image and developed as before. After the stop bath and wash it was soaked for l /zmlnutes at 20 C. in the same as used in example 6. After washing in hot water, a negative record was obtained.

EXAMPLE 9 To 200 ml. 10% gelatin solution was added 100 ml. ofa 12% aqueous dispersion of carbon black (Sterling MTFF), followed by 3 ml. Teepol X and 0 .2 g. di-(p-nitrophenyll dithiodiglycollate as prepared in Preparation ll dissolved in 20 ml. Z-ethoxyethanol. After adjusting the pH to 7, the volume was adjusted to 500 ml. and the solution coated on clear flexible base at 20 feet per minute and 35 C. When dry the coating had a density greater than 4 and it was then stored for 2 days at 125 F. and 68% RH to accelerate the cross-linking reaction. A slow chlorobromide emulsion was then applied to the coating, to give a silver coating weight of 22 mg/dm A strip of this material was exposed to a step wedge. developed as before, and then soaked in the same solution as used in example 8. After washing in hot water a positive record was obtained.

EXAMPLE I I) A strip of the material described in example 5 was exposed to a step wedge, developed for 2 minutes in a Phenidonehydroquinone developer at 20 C., washed for 5 minutes and then soaked in a 5% aqueous sodium carbonate solution containing 2.3 g./liter of 2 mercaptoethylamine hydrochloride for 2 minutes at 24 C. After washing in hot water a positive record was made.

EXAMPLE ii A strip of the material described in example 5 was treated exactly as in example except that the sodium carbonate solution contained 1.56 g./liter of Z-mercaptoethanol. After washing in hot water a positive record was obtained.

EXAMPLE 12 A strip of the material described in example 5 was exposed to a line image, then developed as before, and after washing for 5 minutes it was soaked in a 5% sodium carbonate solution containing 3 g./liter thiomalic acid for 3 minutes at 20 C. After washing in hot water a positive record was obtained.

EXAMPLE 13 A strip of the material described in example 5 was exposed to a line image and then developed as before, and after washing for 5 minutes it was soaked in a 5% sodium carbonate solution containing 2.2 g./liter l-thioglycerol for 2 minutes at 20 C. After washing in hot water a positive record was obtained.

A second strip of the same material, also exposed to a line image, was processed in exactly the same manner except that the time of immersion in the thioglycerol solution was 15 seconds. After washing in hot water a negative record was ob tained.

EXAMPLE 14 A strip of the material described in example 5 was exposed to a line image and was then developed for 2 minutes at C. in the same Phenidone-hydroquinone developer as before, to which had been added 3.14 g./liter cysteine hydrochloride; the pH was adjusted back to its original value of 10.5. After washing in hot water a negative record was obtained.

The words Phenidone and Teepol used in the Examples are Registered Trade Marks l claim as my invention:

1. Photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cross-linked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer.

2. Photographic material according to claim 1 wherein the gelatin layer comprises an inert pigment.

3. Photographic material according to claim 2 wherein the inert pigment is carbon black.

4. A process for the preparation of photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is crosslinked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decrosslinkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer which comprises coating a layer of thiolated gelatin on a support and subjecting the layer to aerial oxidation to affect cross-linking of thiol groups to form disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and thereafter coating a layer of a gelatin silver halide emulsion thereon.

5. A process for the preparation of photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is crosslinked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decrosslinkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer which comprises treating an a ueous gelatin solution with a gelatin cross-linking agent of t iie formula R -S -S -R' where R and R are each selected from groups capable of reacting with gelatin so as to cross-link the gelatin by the formation therein of disulfide linkages, coating the resulting mixture on a support and thereafter coating a layer of a gelatin silver halide emulsion thereon.

6. A method of producing an image of an original subject which comprises imagewise-exposing photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is crosslinked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decrosslinkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer developing the material in a nontanning silver halide developing solution and simultaneously or subsequently treating the material with an alkaline solution ofa mercaptan for a predetermined period of time, and washing the material in hot water to remove the areas of decross-linked gelatin and the silver halide emulsion adherent thereto.

7. A method according to claim 6 wherein the material after exposure is developed in a nontanning developing solution and then treated with an aqueous alkaline solution of a mercaptan compound.

8. A method of producing a negative image of an original subject which comprises imagewise-exposing photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cross-linked by the presence therein ofdisulfide linkages suffcient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer, developing the material in a nontanning silver halide developing solution, removing the undeveloped silver halide from the material by fixing it out, bleaching the developed silver to silver halide, treating the material with an aqueous alkaline solution of a mercaptan compound and washing the material in hot water thereby to remove the silver halide emulsion from the material and the gelatin from the unexposed areas of the material, 

2. Photographic material according to claim 1 wherein the gelatin layer comprises an inert pigment.
 3. Photographic material according to claim 2 wherein the inert pigment is carbon black.
 4. A process for the preparation of photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cRoss-linked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer which comprises coating a layer of thiolated gelatin on a support and subjecting the layer to aerial oxidation to affect cross-linking of thiol groups to form disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and thereafter coating a layer of a gelatin silver halide emulsion thereon.
 5. A process for the preparation of photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cross-linked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer which comprises treating an aqueous gelatin solution with a gelatin cross-linking agent of the formula R -S -S -R'' where R and R'' are each selected from groups capable of reacting with gelatin so as to cross-link the gelatin by the formation therein of disulfide linkages, coating the resulting mixture on a support and thereafter coating a layer of a gelatin silver halide emulsion thereon.
 6. A method of producing an image of an original subject which comprises imagewise-exposing photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cross-linked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer developing the material in a nontanning silver halide developing solution and simultaneously or subsequently treating the material with an alkaline solution of a mercaptan for a predetermined period of time, and washing the material in hot water to remove the areas of decross-linked gelatin and the silver halide emulsion adherent thereto.
 7. A method according to claim 6 wherein the material after exposure is developed in a nontanning developing solution and then treated with an aqueous alkaline solution of a mercaptan compound.
 8. A method of producing a negative image of an original subject which comprises imagewise-exposing photographic material which comprises a support having coated on one side thereon, in order, a non-light-sensitive gelatin layer which is cross-linked by the presence therein of disulfide linkages sufficient to withstand immersion in hot water, said layer being decross-linkable by treatment with an aqueous alkaline solution of a mercaptan, and an unhardened gelatin silver halide emulsion layer, developing the material in a nontanning silver halide developing solution, removing the undeveloped silver halide from the material by fixing it out, bleaching the developed silver to silver halide, treating the material with an aqueous alkaline solution of a mercaptan compound and washing the material in hot water thereby to remove the silver halide emulsion from the material and the gelatin from the unexposed areas of the material. 